The present invention relates to ultraviolet-curing resin compositions, and more particularly, to ultraviolet-curing resin compositions suitable for use as secondary cladding materials for a core of an optical fiber and as buffer materials for optical fiber units.
Optical fibers are comprised of a core and a sheath formed around the core. The core and sheath are formed from two kinds of glass or two kinds of plastic, each having a different refractive index. One or more cladding layers is formed around the glass or plastic core. The primary cladding layer is formed directly on the core. A secondary cladding may be formed directly around the primary cladding or it may be produced by forming the secondary cladding on the primary cladding via a buffer layer.
Regardless of the structure, the secondary cladding typically is formed by extrusion-molding a thermoplastic resin, such as a polyamide or a polyethylene and, as a result, the drawing rate of the optical fiber core is limited. As a result, the mass production of optical fibers is limited.
Although the formation of a secondary cladding from an ultraviolet-curing resin composition has been proposed, no secondary cladding material having the combination of satisfactory modulus of elasticity, elongation and resistance to moist heat has been found.
Optical fibers are typically used in the form of optical fiber units, which are an assembly of optical fiber cores arranged around a central member with a buffer material separating the central member and the optical fiber cores. A copper wire is used as the central member and the clad glass or plastic fibers described above are the cores. Units or bundles of this type are particularly useful in communication cables.
Plastic yarns made from a thermoplastic resin have been used as the buffer material. Such yarns suffer from a low molding rate which makes manufacturing cost high. The use of an ultraviolet-curing resin as a buffer material has been proposed to increase the molding rate and to improve workability.
A secondary cladding material and a buffer material must be compatible, but their use requires that their physical properties be different. A secondary cladding material must have a pressure resistance sufficient to protect the first cladding material. When the modulus of elasticity is too low, i.e., below 25 kg/mm.sup.2, the cladding does not impart sufficient strength to the fiber and the fiber is easily broken. When the modulus is too high, i.e., greater than 75 kg/mm.sup.2, there is insufficient shock absorption. In addition, the cladding must be sufficiently flexible so as not to break when tension or bending force is applied. An elongation of at least 15% is required.
A buffer material must be softer and more flexible than the secondary cladding material. Its primary function is to absorb shock, and, therefore, a modulus of elasticy between 1 and 25 kg/mm2 is desired. The buffer material must have an elongation of at least 40% to allow the cable bundle to be routed through ducts and around corners.
However, the ultraviolet-curing resin compositions used to date have poor moisture resistance, elongation and elasticity, although it does have an improved workability. No buffer material comprising an ultraviolet-curing resin composition which satisfies all of the above requirements has been found yet.
For example, an epoxy acrylate composition has a defect that its modulus of elasticity is too high and elongation is insufficient.
Urethane acrylate compositions have a problem that its resistance to moist heat and its photo-curing rate are low.